SQL查询：求每个组最大值，原来是这样写的（SQL分组求最值有哪些坑）
背景介绍：
这是我9月份的一道SQL面试题，当时我答错了，于是写了这篇博客总结一下
表和数据如下
CREATE TABLE `user` (
`uid` int(11) NOT NULL AUTO_INCREMENT COMMENT '用户id',
`dept_id` int(11) DEFAULT NULL COMMENT '分组id',
`name` varchar(255) DEFAULT NULL COMMENT '用户名',
`score` int(11) DEFAULT NULL COMMENT '分数',
PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8mb4;
-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES ('1', '1', 'a', '12');
INSERT INTO `user` VALUES ('2', '1', 'aa', '13');
INSERT INTO `user` VALUES ('3', '1', 'aaa', '99');
INSERT INTO `user` VALUES ('4', '2', 'bb', '40');
INSERT INTO `user` VALUES ('5', '3', 'c', '60');
INSERT INTO `user` VALUES ('6', '2', 'b', '12');
INSERT INTO `user` VALUES ('7', '2', 'bbb', '1');
需求
按照dept_id进行分组，求每个分组的score的最大值
我的答案
SELECT * FROM `user` GROUP BY dept_id HAVING score = MAX(score)
运行结果
mysql> SELECT * FROM `user` GROUP BY dept_id HAVING score = MAX(score);
+-----+--------+-------+-------+
| uid | dept_id | name | score |
+-----+--------+-------+-------+
| 4 | 2 | bb | 40 |
| 5 | 3 | c | 60 |
+-----+--------+-------+-------+
2 rows in set
SQL语句执行顺序
1. FROM <表名> # 选取表，将多个表数据通过笛卡尔积变成一个表。
2. (left/right) JOIN #指定join，用于添加数据到on之后的虚表中，例如left join会将左表的剩余数据添加到虚表中
3. ON <筛选条件> # 对笛卡尔积的虚表进行筛选
4. WHERE <筛选条件> # 对上述虚表进行筛选
5. GROUP BY <分组条件> # 分组
6. HAVING <分组筛选> # 用于having子句进行判断，在书写上这类聚合函数是写在having判断里面的
7. SELECT #返回数据列表
8. DISTINCT （或者其它聚合函数）
9. ORDER BY 排序
问题分析
先看一下直接查询表中全部数据
mysql> SELECT * FROM `user`;
+-----+--------+-------+-------+
| uid | dept_id | name | score |
+-----+--------+-------+-------+
| 1 | 1 | a | 12 |
| 2 | 1 | aa | 13 |
| 3 | 1 | aaa | 99 |
| 4 | 2 | bb | 40 |
| 5 | 3 | c | 60 |
| 6 | 2 | b | 12 |
| 7 | 2 | bbb | 1 |
+-----+--------+-------+-------+
7 rows in set
再看一下分组查询的结果
mysql> SELECT * FROM `user` GROUP BY dept_id;
+-----+--------+-------+-------+
| uid | dept_id | name | score |
+-----+--------+-------+-------+
| 1 | 1 | a | 12 |
| 4 | 2 | bb | 40 |
| 5 | 3 | c | 60 |
+-----+--------+-------+-------+
3 rows in set
分组后加上Having MAX
mysql> SELECT * FROM `user` GROUP BY dept_id HAVING score = MAX(score);
+-----+--------+-------+-------+
| uid | dept_id | name | score |
+-----+--------+-------+-------+
| 4 | 2 | bb | 40 |
| 5 | 3 | c | 60 |
+-----+--------+-------+-------+
2 rows in set
最终分析
每个分组的最大值分别为：
- 1：90
- 2：40
- 3：60
由于直接运行SELECT * FROM user GROUP BY dept_id返回的结果中，分组为2，3的数据中，score是2，3组的最大值，加上Having max过滤后任然没有被过滤掉，而分组1返回的数据中，score不是最大值，被过滤掉了
另一个看起来正确但实际上是错误的答案
mysql> SELECT uid,dept_id,`name`,MAX(score) FROM `user` GROUP BY dept_id;
+-----+--------+-------+------------+
| uid | dept_id | name | MAX(score) |
+-----+--------+-------+------------+
| 1 | 1 | a | 99 |
| 4 | 2 | bb | 40 |
| 5 | 3 | c | 60 |
+-----+--------+-------+------------+
3 rows in set
不仔细看是不是以为答案正确了，但是，有没有发现uid和score对不上啊
正确答案
SQL语句
SELECT * FROM `user` as u1 WHERE u1.score =(
SELECT MAX(score) FROM `user` as u2 WHERE u1.dept_id=u2.dept_id);
运行结果
mysql> SELECT * FROM `user` as u1 WHERE u1.score =(
SELECT MAX(score) FROM `user` as u2 WHERE u1.dept_id=u2.dept_id);
+-----+--------+-------+-------+
| uid | dept_id | name | score |
+-----+--------+-------+-------+
| 3 | 1 | aaa | 99 |
| 4 | 2 | bb | 40 |
| 5 | 3 | c | 60 |
+-----+--------+-------+-------+
3 rows in set
最终的答案参考了这篇博客，地址如下：https://www.cnblogs.com/lakeliu/p/11943946.html